The present invention is particularly applicable for monitoring the actual heating characteristics of an induction heating system as the system is heating a metal workpiece while it is stationary and it will be described with particular reference thereto: however, as discussed in this application, the invention has broader applications and may be employed for monitoring the actual heating cycle of successive heating cycles employing an induction heating coil encircling a metal workpiece which is stationary or axially movable through the inductor.
For many years the induction heating industry has been considering the possibility of controlling induction heating systems by a variety of non-destructive sensors which could be interfaced with appropriate microprocessors or programmable controllers to either control the actual processing of a workpiece or determine when such workpiece was defective. Such "smart" control systems for induction heating equipment have been primarily incorporation of pyrometers, heat sensors and watt meters to control the power applied to the workpiece during processing. This type of integrated control has been primarily applicable for induction heating of long wires or strands. It was not applied to production processing of discrete workpieces and inductively heated for quench hardening in the automotive industry, or other consumer product industries. To control discrete workpiece heating in mass production induction heating systems, there has been really few successful control mechanisms for in-process monitoring. As disclosed in Balzer U.S. Pat. No. 4,618,125, it is possible to pass a previously induction heated quench hardened camshaft through or with respect to an eddy current sensing device to determine whether or not the hardening operation is in accordance with a preselected plan or pattern. The adaptation of eddy current principles and technology to evaluating the quality of a previously processed part or workpiece, including one or more selectively hardened portions, was pioneered by assignee of the present application and is disclosed in the prior patent together with the previously mentioned copending patent application on processing hardened camshafts. As is well known, the eddy current sensing arrangement, as disclosed in the Balzer patent, can only detect the history of an inductively heated and quench hardened workpiece, whether heating is done with the workpiece stationary or movable, such as a camshaft hardening process.
When developing the concept of moving an eddy current detector coil around a previously hardened workpiece having axially spaced differences in hardness and metallurgical characteristics, a variety of systems could be employed to pulse an eddy current driving coil and to evaluate the reflected pulses from the eddy current pick-up or sensing coil. One of such systems is illustrated in FIGS. 12 and 13 of copending U.S. application Ser. No. 859,348, filed May 5, 1986. Another system which could be used to drive the eddy current coil and detect the electromagnetic characteristics of the workpiece along its length by an encircling eddy current detection coil is illustrated in Mordwinkin U.S. Pat. Nos. 4,059,795 and 4,230,987. These two patents, which are incorporated by reference herein, are directed to the use of eddy current technology to determine metallurgical characteristics of a stationary metal specimen primarily for the purpose of determining the identity of the specimen, much like spectrum analysis. This eddy current processing circuit and concepts illustrated in the Mordwinkin patents can be employed for the purpose of sensing the electromagnetic characteristics along the length of a previously hardened camshaft, as illustrated in Balzer U.S. Pat. No. 4,618,125. Indeed other eddy current driving and sensing circuits can be employed for detecting the electromagnetic characteristics of a workpiece movable through a pair of coils after the workpiece has been inductively heated and then quench hardened in a manner similar to a camshaft. Such detection will involve both physical characteristics of the workpiece, such as geometry which cannot change during hardening, and metallurgical characteristics such as hardness, grain size, grain phase, etc.
When such eddy current technology is applied to in-process use, in conjunction with induction heating, it has been found by assignee to be quite beneficial and has been, or is, in the process of being widely accepted by industry, especially the automotive and consumer product industries. By these non-destructive testing procedures previously hardened portions of a complex workpiece can be analyzed to determine whether or not the workpieces conform to a preselected pattern and/or characteristics ascribed to acceptable workpieces however, like many advances in the induction heating art, this advance in non-destructive testing to monitor the actual performance of a complex induction heating process or system has several disadvantages. A special driving coil and sensing coil must be employed. A special work station must be provided when space for such a station is usually at a premium. The eddy current testing system requires additional processing time, since the eddy current testing of the previously hardened portions, even when done by scanning, requires cycle time. Eddy current equipment also requires a power source for energizing the driving coil, which power source adds further cost, expense and maintenance difficulties to the total induction heating system or equipment.
In view of this state of the art, assignee of the present application has been seeking an arrangement for in-process monitoring of induction heating equipment, without requiring destructive testing and without the disadvantages concomitant with prior efforts, albet somewhat successful, to apply eddy current technology to the induction heating field.